The present invention relates, in general, to the field of electronic devices and apparatus and methods for retaining the same. More particularly, the present invention is of especial utility in retaining an electronic device such as a light emitting diode (LED) having an encapsulated body portion which must be physically maintained in a generally fixed position relative to an indicator panel yet simultaneously allow for the thermal expansion and contraction of the device body and leads thereof without placing undue stress on its lead/die interface.
In the mounting of certain electronic devices, and in particular LEDs, a specific height spacing for the device upper end is desired to ensure a generally uniform plane surface between adjacent devices and an indicator panel. Unfortunately, because the encapsulated body portion of an LED is made of poured epoxy, the physical height of the device itself from bottom end to top end can vary by up to 40 mils (1.016 mm) rendering the bottom end, for the most part, useless as a mounting reference level. This fact makes the maintenance of a controlled height to the top of the LED very difficult when dealing with generally rectangular LEDs. To deal with this variability in device body height, certain cylindrical LEDs have an integral annular flange around their periphery to provide a constant dome-to-flange height whereby the device may be mounted through a panel hole such that the variability as to the overall device height becomes unimportant. However, these flanged devices, therefore, require a more costly and complex mounting structure than those having a generally rectangular cross section.
Another problem which must be dealt with in the mounting of LEDs by rigidly fixing the device body height to maintain height spacing is that of thermal expansion of the epoxy or plastic body with respect to the device leads. Under normal thermal conditions such as soldering, device operation or mere proximity to other heat-producing components, a great deal of stress may be experienced at the lead/die interface of the device. This stress results from the fact that the plastic device body has a coefficient of thermal expansion which may be up to ten times that of the device leads. Therefore, rigidly affixing the device body height to fix the plane of the LED top can result in premature device failure at the lead/die interface due to this stress. In light of this fact, it has been the practice to use lead bending techniques to ensure that sufficient lead length is maintained between a device body portion and the printed circuit board to which it is soldered such that the device body and leads can expand relatively independently and thereby reduce stress on the lead/die interface. As is easily perceived however, this technique introduces at least another extra and undesirable assembly step and further complicates the expeditious insertion of the LED leads to a printed circuit board by, of necessity, introducing a variable into the LEDs intralead spacing.
It is therefore an object of the invention to provide an improved retaining apparatus and method for an electronic device.
It is further an object of the present invention to provide an improved retaining apparatus and method for an electronic device which allows for uniform height spacing of the device despite variations among devices in their dimensional parameters.
It is still further an object of the present invention to provide an improved retaining apparatus and method for an electronic device which does not require lead forming to compensate for thermal stress at a lead/die interface.
It is still further an object of the present invention to provide an improved retaining apparatus and method for an electronic device which is adaptable to devices having a generally uniform cross section of any geometrical shape.
It is still further an object of the present invention to provide an improved retaining apparatus and method for an electronic device which may be fabricated with lead guides for ease of insertion and subsequent soldering of the device leads to a printed circuit board.